A mower unit includes a plurality of blades for securing a greater mowing width, so that rotational axis of the plurality of blades extends upwardly of a mower deck. For rotation of the blades, power of a power generating unit such as an engine is utilized. A mower unit disclosed in U.S. Patent Application Publication No. 6176071 employs a belt power transmission mechanism for power transmission, including a first belt transmission mechanism for transmitting rotation of an output pulley of a power generating unit to one blade rotational shaft and a second belt transmission mechanism for transmitting rotation of this blade rotational shaft to another blade rotational shaft. An idler pulley is provided for applying tension by means of a spring to both the first belt transmission mechanism and the second belt transmission mechanism. With provision of the two belt transmission mechanisms, stable power transmission is sought for. However, the presence of the two belt transmission mechanisms is problematic in the respects of cost and space required therefor.
A grass mower machine having only one belt transmission mechanism is disclosed in U.S. Patent Application Publication No. 8763355. According to this grass mower machine, a length of belt entrained around an output pulley mounted on an engine output shaft extends to a mower unit which is disposed on a front side of a machine body and this belt is then changed in direction firstly by two idler pulleys (a fixed idler pulley and a pivotal idler pulley) to a left-right direction of the machine body to be routed around a plurality of blade pulleys. In this arrangement, the distance between the output pulley and the idler pulleys for change of direction is relatively short. So, in the event of lifting up/down of the mower unit by a large stroke, the tilting of the belt portion entrained around the output pulley is large, so there is the possibility of the load applied to its belt portion becoming significant.